This invention relates generally to magnetic resonance imaging (MRI) of an object having different chemical species therein, such as fat and water, and more particularly the invention relates to species imaging in the presence of magnetic field (Bo) heterogeneity.
Reliable and uniform fat suppression is essential for accurate diagnoses in many areas of MRI. This is particularly true for sequences such as fast spin-echo (FSE), steady-state free precession (SSFP) and gradient echo (GRE) imaging, in which fat is bright and may obscure underline pathology. Although conventional fat saturation may be adequate for areas of the body with relative homogeneous Bo field, there may be many applications in which fat saturation routinely fails. This is particularly true for extremity imaging, off-isocenter imaging, large field of view (FOV) imaging, and challenging areas such as the brachial plexus and skull base, as well as many others. Short-TI inversion recovery (STIR) imaging provides uniform fat suppression, but at a cost of reduced signal-to-noise ratio (SNR) and mixed contrast that is dependent on T1. This latter disadvantage limits STIR imaging to T2 weighted (T2W) applications, such that current T1 weighted (T1W) applications rely solely on conventional fat-saturation methods. Another fat suppression technique is the use of spectral-spatial or water selective pulses; however, this method is also sensitive to field inhomogeneities.
“In and Out of Phase” Imaging was first described by Dixon in “Simple Proton Spectroscopic Imaging”, Radiology (1984) 153:189-194, and was used to exploit the difference in chemical shifts between water and fat and in order to separate water and fat into separate images. Glover et al. further refined this approach, described in Glover G., “Multipoint Dixon Technique for Water and Fat Proton and Susceptibility Imaging”, Journal of Magnetic Resonance Imaging (1991) 1:521-530, with a 3-point method that accounts for magnetic field inhomogeneities created by susceptibility differences. This method was applied with FSE imaging by acquiring three images with the readout centered at the spin-echo for one image and symmetrically before and after the spin-echo in the subsequent two images.